Controversial 20mph speed limits in residential streets may not bring any significant improvement in road safety, a report published by the Department for Transport has found.

Towns and cities across Britain are planning to introduce 20mph limits, with Government backing, claiming that they will cut the toll of deaths on the road.

However, an analysis of the UK's first city-wide scheme - in which the limit was lowered from 30mph to 20mph on all residential streets in Portsmouth, at a cost of £500,000 - found that it has not brought any significant reduction in the number of accidents.

The number of people killed or seriously injured on affected roads actually went up, not down, after the limit was lowered.

Motorists' groups said the findings cast doubt on the case for city-wide 20mph schemes.

Paul Watters, head of public affairs at the AA, said: "By just whacking up signs everywhere you are not going to change things dramatically.

"We support targeted and tailored 20mph zones where they are really needed, not a blanket implementation across a whole city.

"Sometimes the limits can be problematic because bus journey times are affected, and also there is an impact for delivery firms and everyone else driving in the city."

Newcastle-upon-Tyne, Oxford, Edinburgh and Bristol have all introduced 20mph limits in their city centres since the Portsmouth scheme began in 2007.

In Portsmouth, the new, lower speed limit applies to all vehicles, at all times, on 94 per cent of the city's streets. It is not enforced by speed cameras or road humps, but relies on drivers to obey limit signs.

The analysis, carried out by the consultants Atkins on behalf of the DfT, found that prior to the reduction in the limit, an average of 18.7 people per year were killed or seriously injured on the streets covered.

After the reduction this rose to 19.9 per year.

Taking into account people with less serious injuries, overall road casualties fell by 22 per cent after the scheme was introduced.

However, over the same time period, there was a nationwide fall in road casualties of 14 per cent. The Atkins report concluded "casualty benefits greater than the national trend have not been demonstrated".

Motorists' average speeds reduced by 1.3mph, from 19.8mph to 18.5mph, as a result of the scheme, according to the report, which tested speeds at 223 locations across the city.

However, independent statisticians criticised the way that the figures had been calculated, and said the true reduction in speed could be smaller.

Professor Stephen Senn, an expert in statistics at the University of Glasgow, said: "The design of the report is very bad. Various statistical terms are used incorrectly and they've probably used the wrong statistical test.

"They haven't got a control group, which is pretty basic, and without which it is pretty naive to jump to conclusions."

Previous studies have found that 20mph "zones", in which traffic-calming measures are deployed in addition to speed limit signs, produce bigger reductions in drivers' average speeds.

Norman Baker, the Liberal Democrat transport minister, has given his backing to 20mph limits.

In an interview in June he said: "For a child being hit at 30mph and 20mph is the difference between life and death.

"But this is also about making out town centres more attractive places to live and work, and reducing carbon emissions by encouraging people to cycle or walk."

However, the push for new 20mph roadsigns flies in the face of separate Government guidance to local authorities which encourages them to keep street clutter to a minimum.

The Portsmouth report stated that "sign clutter has been a problem at some locations".

Research has found that when pedestrians are hit by a vehicle at 20mph only one in 40 dies, compared with one in five at 30mph. However only 15 per cent of fatal crashes and 5 per cent of all accidents are caused by speeding.

Idris Francis, a retired company director who lives near Portsmouth and has previously spoken out against speed cameras, said the scheme had not worked.

"They are trying to manipulate the figures to show what they want," he said.

"But in many areas speeds actually increased. It is worrying that the limits might be changing the behaviour of people for the worse and making some roads more dangerous."

A spokesman for the Department for Transport said: "This report by an independent contractor on the Portsmouth city council scheme is one of several research documents available to help councils decide whether they want to put in place 20 mph zones or limits on their roads.

"It is up to local authorities to make these decisions using their knowledge of local roads and in consultation with local communities."

An Atkins spokesman added: "Industry standard methodology was used in the preparation of this report. All findings were peer-reviewed internally and by the DfT prior to publication."